Wow, that sucks, but I'm not surprised. Many people are really starting to abuse the carry-on luggage thing, bringing big bags, plus a large "laptop" bag. This is either in addition to their checked bag(s), or instead of. Either way, on almost every Spirit flight I've been on the overhead bins are completely full and several carry-on's end up being checked.
This is what happens when people start to abuse things, and Spirit likely won't be the first or only low budget airline to do this.
I'll continue to fly with them until they are not the lowest price option that I can find anymore ... until then ... I guess I'll be paying a little more ... :-(